Method of manufacturing expendable salt core for casting and expendable salt core for casting

ABSTRACT

A die casting method for forming an expandable salt core includes, first, a salt mixture containing at least a potassium salt and a sodium salt is melted by heating to obtain a melt. Subsequently, the temperature of the melt is decreased to set the melt in a semi-solidified (solid-liquid coexisting) state. The melt in the semi-solidified state is injected into a metal mold under a high pressure and solidified. After solidification, the obtained expendable salt core is taken out of the metal mold.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a water soluble expendable salt core, and an expendable salt core.

2. Description of the Related Art

As is well known, casting methods such as aluminum high pressure die casting (HPDC) is a technique that injects a molten aluminum alloy into a metal mold at high speed under a high pressure in order to cast a near-net-shaped structure. In this casting, when molding a cast product having a hollow structure, e.g., a water cooling water jacket in a closed-deck type cylinder block of an internal combustion engine, an expendable core is used. The expendable core used in these instances must have a strength that can withstand a high pressure and high temperature because it may be subject to a large impact or impulse force fluctuation upon collision of a molten metal injected from the gate at high speed mold filling and because a high static compressive casting pressure is applied until solidification completion. After casting, the expendable core is removed from the cast product. When the cast product has a complicated internal structure, if a commonly used phenol resin bonded sand core is used as the expendable core, it is not easy to remove. In contrast to this, a water soluble expendable salt core is available as the expendable core that can be removed by dissolution with, e.g., high-temperature water (see Japanese Patent Laid-Open No. 48-039696, Japanese Patent Laid-Open No. 50-136225, and Japanese Patent Laid-Open No. 52-010803).

The expendable salt core described above is formed by using a salt mixture of, e.g., sodium carbonate (Na₂CO₃), potassium chloride (KCl), and sodium chloride (NaCl), melting these components, and molding. Hence, a high static compressive casting pressure resistance is obtained, and workability and stability of dimension accuracy in casting are improved.

However, when an expendable fused salt core is formed by melting a salt and casting the formation of a shrinkage cavity, micro-porosity, small heat cracks, and the like are caused in the salt core due to a change in volume such as solidification shrinkage occurring in the solidification process. It is therefore not easy to mold the expendable fused salt core so that it will precisely conform to the mold. Also, depending on the composition of the respective components, the resultant melting point is 700° C. or more, which is not suitable for molding by melting. In this manner, with the prior art, an expendable fused salt core cannot be manufactured easily by casting using a molten salt.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention have been made to solve the above problems, and are able to facilitate a manufacture of a water soluble expendable salt core which is formed of a salt cast product obtained by molding after melting salts such as sodium and potassium.

A method of manufacturing an expendable salt core according to a preferred embodiment of the present invention comprises at least a first step of heating a salt mixture containing at least a potassium salt and a sodium salt to form a melt in a solid-liquid coexisting state in which a solid phase and a liquid phase coexist, a second step of charging the melt in the solid-liquid coexisting state into a core mold, and a third step of solidifying the melt in the mold to mold an expendable salt core for casting. Therefore, at the point when the mold is charged with the melt, the melt is partly solidified.

An expendable salt core according to the present invention is molded by heating a salt mixture containing at least a potassium salt and a sodium salt to provide a melt in a solid-liquid coexisting state in which a solid phase and a liquid phase coexist, charging the melt in the solid-liquid coexisting state into a core mold, and solidifying the melt in the mold. This expendable salt core for casting includes, e.g., a core to mold a water jacket for water cooling a closed-deck type cylinder block of engine.

According to preferred embodiments of the present invention, an expendable salt core is cast using a melt in a solid-liquid coexisting state. Therefore, a water soluble expendable salt core is made of a salt cast product, which is molded by melting salts such as sodium and potassium, and can be easily manufactured.

Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cylinder block which is cast using an expendable salt core according to a preferred embodiment of the present invention.

FIG. 2 is a photograph obtained by a metallographic microscope (optical microscope) which shows the state of the solidification structure of a salt core according to a preferred embodiment of the present invention.

FIG. 3 is a graph showing the temperature dependence of the solid phase ratio of a melt in a semi-solidified state.

FIG. 4 is a photograph of a solidification structure obtained by a scanning electron microscope when a salt mixture with a composition containing a large amount of chloride is melted, molded, and solidified without stirring.

FIG. 5 is a photograph of a solidification structure obtained by a scanning electron microscope when a salt mixture with a composition containing a large amount of carbonate is melted, molded, and solidified without stirring

FIG. 6A is a graph showing the bending strengths of bending test pieces of sample Nos. 1 to 9.

FIG. 6B is a graph showing the bending strengths of bending test pieces of sample Nos. 10 to 12.

FIG. 6C is a graph showing the bending strengths of bending test pieces of sample Nos. 13 to 17.

FIG. 7A is a graph showing the bending strengths of bending test pieces of sample Nos. 18 to 23.

FIG. 7B is a graph showing the bending strengths of bending test pieces of sample Nos. 24 to 27.

FIG. 8 is a phase diagram showing the relationship among the cation ratio of potassium ions and sodium ions, the anion ratio of carbonate ions and chloride ions, and the liquidus temperature thereof.

FIG. 9A is a view showing the state of a test piece used in a bending strength measurement.

FIG. 9B is a sectional view showing a portion of the test piece used in a bending strength measurement.

FIG. 10 is a view for explaining the bending strength measurement.

FIG. 11 is a photograph that explains pressure measurement portions in the cavity of an expendable salt core during injection molding.

FIG. 12 is a graph that shows the measurement result of the pressure in the cavity of the expendable salt core during injection molding.

FIG. 13 is a perspective view of another cylinder block which is cast using an expendable salt core according to a preferred embodiment the present invention.

FIG. 14 is a photograph of the expendable salt core shown in FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinafter with reference to the drawings. First, how an expendable salt core for casting according to a preferred embodiment of the present invention is used will be described with reference to FIG. 1. FIG. 1 is a partially cutaway perspective view of a closed-deck type cylinder block which is cast using the expendable salt core according to a preferred embodiment of the present invention. Referring to FIG. 1, reference numeral 1 denotes a closed-deck type cylinder block of engine, which is made of, for example, an aluminum alloy and cast using an expendable salt core 2 as an expendable salt core according to a preferred embodiment the present invention. The cylinder block 1 is a portion of a water cooling 4-cycle 4-cylinder engine for a motorcycle which is molded into a predetermined shape by high pressure die casting (HDPC), for example.

The cylinder block 1 shown in FIG. 1 integrally includes four cylinder bores 3, a cylinder body 4 having the cylinder bores 3, and an upper crank case 5 extending downward from the lower end of the cylinder body 4. A lower crank case (not shown) is attached to the lower end of the upper crank case 5. The upper crank case 5, together with the lower crank case, rotatably, axially supports a crank shaft (not shown) through a bearing.

The cylinder body 4 is a so-called closed-deck-type cylinder body, and has a water jacket 6 which is provided in it using the expendable salt core 2. The water jacket 6 is arranged to include a cooling water passage forming portion 7, cooling water inlet port 8, main cooling water passage 9, and communication passage 10. The cooling water passage forming portion 7 projects on one side of the cylinder body 4 and extends in the direction in which the cylinder bores 3 line up with one another. The cooling water inlet port 8 is provided in the cooling water passage forming portion 7. The main cooling water passage 9 is provided to communicate with a cooling water distribution passage (not shown) provided in the cooling water passage forming portion 7 and cover all the cylinder bores 3. The communication passage 10 extends upward in FIG. 1 from the main cooling water passage 9 and opens to a mating surface 4 a with respect to a cylinder head (not shown) at the upper end of the cylinder body 4.

The water jacket 6 described above supplies cooling water flowing in from the cooling water inlet port 8 to the main cooling water passage 9 around the cylinder bores through the cooling water distribution passage, and guides the cooling water from the main cooling water passage 9 to a cooling water passage in the cylinder head (not shown) through the communication passage 10. Since the water jacket 6 is provided in this manner, the cylinder body 4 is covered with the ceiling wall (the wall that provides the mating surface 4 a) of the cylinder body 4 except that the communication passage 10 of the water jacket 6 opens to the mating surface 4 a at the upper end to which the cylinder head is connected. Hence, a closed-deck-type arrangement is provided.

The expendable salt core 2 has a shape identical to that obtained by integrally connecting the respective portions of the water jacket 6. In FIG. 1, the cylinder body 4 is partly cut away to facilitate understanding of the shape of the expendable salt core 2 (the shape of the water jacket 6).

The expendable salt core 2 according to this preferred embodiment is formed using a plurality of types of salts such as sodium carbonate, sodium chloride, and potassium chloride by, e.g., die casting in a solid-liquid coexisting state such as a semi-solidified state into the shape of the water jacket 6. The expendable salt core 2 may be formed by heating a salt mixture containing at least a potassium salt and a sodium salt to form a melt in a solid-liquid coexisting state in which a solid phase and liquid phase coexist, charging the melt into a core mold, and solidifying the melt in the mold. A method for manufacturing the expendable salt core 2 will be described later in detail.

Note that the expendable salt core 2 can be formed by a casting method other than die casting, e.g., gravity casting. In forming the expendable salt core 2 which employs die casting, first, a mixture consisting of a plurality of salts (to be described later) is melted by heating to obtain a melt. Then, the temperature of the melt is decreased to set the melt in a semi-solidified (solid-liquid coexisting) state. The melt in the semi-solidified state is injected into an expendable salt core forming metal mold under a high pressure and solidified. After solidification, the obtained expendable salt core 2 is taken out from the mold.

As shown in FIG. 1, in the expendable salt core 2, the passage portion 2 a which provides the cooling water inlet port 8 and the cooling water distribution passage, an annular portion 2 b which surrounds the four cylinder bores 3, and a plurality of projections 2 c extending upward from the annular portion 2 b are provided integrally. The projections 2 c provide the communication passage 10 of the water jacket 6. As is conventionally known, the expendable salt core 2 is supported at a predetermined position in the metal mold (not shown) by a core print (not shown) during casting, and is removed by dissolution with hot water or vapor after casting.

To remove the expendable salt core 2 after casting, the cylinder block 1 may be dipped in a dissolution tank (not shown) which contains dissolving liquid consisting of hydrochloric acid and hot water. When dipping the cylinder block 1 in the dissolving liquid, the passage forming portion 2 a and the projections 2 c exposed to the mating surface 4 a, of the expendable salt core 2 come into contact with the dissolving liquid and dissolve. Dissolving portions expand gradually until all of the portions completely dissolve. In this core removing process hot water or vapor may be sprayed under a pressure from a hole to promote dissolution of the expendable salt core 2 left in the water jacket 6. In the expendable salt core 2, in place of the projections 2 c, a core print may be inserted in portions where the projections 2 c are to be provided.

If hydrochloric acid is used in the process of removing the expendable salt core 2 from the cylinder block 1 as a cast product, carbon dioxide gas will foam. The foam provides a stirring function and promotes effective dissolution. Because the expendable salt core 2 contains potassium carbonate and sodium carbonate, when it is dissolved in water, the resultant water will exhibit alkaline. This alkali state poses problems such as corrosion of the cylinder block 1 which is an aluminum cast product. Regarding this problem, corrosion of the cylinder block can be prevented by adding hydrochloric acid to maintain pH to near 7.

A method of manufacturing the expendable salt core 2 will now be described in detail. First, a case will be described in which the melt is not injected (pressure-injected) under a high pressure as in die casting, but flowed (poured) into the metal mold to manufacture the expendable salt core 2 (gravity casting). To form the expendable salt core 2 of this preferred embodiment, first, sodium carbonate, potassium carbonate, sodium chloride, and potassium chloride are mixed and heated until they are melted, thus preparing a melt of a salt mixture. For example, when the salts are mixed such that a molar ratio XK⁺ (=[K⁺]/([Na⁺]+[K⁺])×100) of potassium ions in the entire cations is 33 mol % and that a molar ratio YCO₃ ²⁻ (=[CO₃ ²⁻]/([CO₃ ²⁻]+[Cl⁻])×100) of carbonate ions in the entire anions is 67 mol %, the salt mixture dissolves at 647° C. For example, the salt mixture described above may be put in an alumina crucible and dissolved in an electric furnace.

Subsequently, when the temperature of the salt mixture accommodated in the crucible reached about 647° C., which is a liquidus temperature or more, the crucible was taken out from the electric furnace and air-cooled. The cooling speed was about 0.3° C. to about 1.2° C. per sec. At this time, the salt mixture in the crucible was stirred with an alumina stirrer with a rotation speed of about 3 rps, and was poured into the metal mold when the temperature of the melt of the salt mixture was about 638° C. When the melt of the salt mixture is about 638° C., the melt is in a semi-solidified state in which the solid phase and liquid phase coexist. The melt in this state is charged into a metal mold for an expendable salt core and solidified, and is taken out from the metal mold after solidification. In the above description, after the salt mixture was heated to set it in a liquid-phase-only state, it was cooled, thus obtaining a melt in a solid-liquid coexisting state. However, the present invention is not limited to this. A melt in the semi-solidified state may also be obtained by heating the salt mixture to a semi-solidification temperature.

The thus obtained expendable salt core 2 had a strength (bending strength) as high as about 21.4 MPa to about 24.6 MPa. The solidification structure of the expendable salt core 2 comprised fine crystal particles as is seen from the photograph shown in FIG. 2 which is obtained through a metallographic microscope. As shown in FIG. 3, in the salt mixture having the above composition, the solid-liquid coexisting temperature range is as wide as about 60° C., and the temperature dependence of the solid phase ratio is small in the solid phase ratio range of about 0% to about 40%. Hence, a melt of a salt mixture with an even solid-liquid coexisting state can be easily obtained. In this manner, according to the manufacturing method of this preferred embodiment, the expendable salt core 2 can be manufactured without strict temperature control or isothermal holding. The range of a temperature where the melt entirely exhibits a solid phase to a temperature where the melt entirely exhibits a liquid phase, in other words, the temperature range where the solid-liquid coexisting state is maintained, changes depending on the composition ratio of the respective components of the salt mixture.

As described above, in the process of cooling the dissolved melt, when the melt temperature reaches the liquidus temperature (melting point) or less, a plurality of solid phase particles are formed and dispersed in the remaining liquid phase. At this time, when stirring the melt in the semi-solidified state, a state can be obtained in which the solid phase is dispersed more evenly in the liquid phase. Note that stirring is not always necessary.

As an example, FIG. 4 shows a photograph of a solidification structure obtained by a scanning electron microscope (SEM) when a salt mixture with a composition of 0 mol % of XK⁺ (=[K⁺]/([Na⁺]+[K⁺])×100) and 10 mol % of YCO₃ ²− (=[CO₃ ²⁻]/([CO₃ ²⁻]+[Cl⁻])×100) is formed by melting and solidified without stirring. With this composition, primary crystals tend to grow into dendrites, and accordingly stirring is preferred. As another example, FIG. 5 shows an SEM photograph of a solidification structure obtained when a salt mixture with a composition of 0 mol % of XK⁺ (=[K⁺]/([Na⁺]+[K⁺])×100) and 70 mol % of YCO₃ ²⁻ (=[CO₃ ²⁻]/([CO₃ ²⁻]+[Cl⁻])×100) is formed by melting and solidified without stirring. With this composition, primary crystals tend to crystallize, and accordingly stirring may not be necessary.

As described above, stirring may be performed, although it is not necessary. Stirring can decrease the temperature distribution in the salt mixture in the solid-liquid coexisting state so that a salt mixture with an even solid phase ratio can be obtained more easily. Also, because stirring can miniaturize and spheroidize solid phase particles in the salt mixture in the solid-liquid coexisting state, it improves moldability. When molding a core with a high solid phase ratio, stirring is preferred. When mechanical stirring is to be performed, a ceramic stirrer which is resistant to corrosion by a molten salt may be employed.

When core molding is initiated in the semi-solidified state having the characteristic features as described above, the amount of solidification shrinkage occurring in the solidification process can be suppressed. Thus, a shrinkage cavity, micro-porosity, small heat crack, or the like which is formed in the expendable salt core can be suppressed. As the amount of solidification shrinkage can be suppressed, the expendable salt core can be more precisely molded to be true to the mold. When casting is initiated in the completely molten state as in the conventional case, the amount of solidification shrinkage is large, so that a shrinkage cavity, micro-porosity, small heat crack, or the like is formed often. The semi-solidification method can suppress these defects, thus improving the strength.

According to the melting molding method, the amount of solidification shrinkage of the core to be molded is larger than the amount of shrinkage of the metal mold. When molding a cylindrical annular core such a water jacket in a cylinder, a shrinkage cavity, micro-porosity, small heat crack, or the like may be formed in the expendable salt core. Depending on the case, the expendable salt core may be broken in the mold. In contrast to this, as described above, use of a melt in the semi-solidified state can reduce the ratio of solidification shrinkage. As a result, a cylindrical annular core such as a water jacket can be formed.

In injection molding using a melt, if the melt is injected with an injection force larger than the machine clamp force of the mold, the melt is scattered from the parting surface, i.e., so-called flushing occurs. In contras to this, in injection molding using a melt in a solid-liquid coexisting state, the leading end of the melt solidifies immediately. Even if the melt is injected with an injection force larger than the mold cavity projected area, flushing does not occur. Therefore, during solidification shrinkage of the melt, a large injection pressure can be applied to replenish the melt, so that the shrinkage cavity can be eliminated. When the melt in the solid-liquid coexisting state is employed, casting can be performed at a temperature lower than that required by the completely molten state. This can improve the workability and decrease the heat load to the casting mold.

As opposed to metals, salt will not oxidize. Even when stirring described above is performed in the atmosphere, no oxide is caught in the melt. Thus, the melt can be stirred easily for a long period of time. Even when molding an annular shape from a semi-solidified state, no oxide skin is formed on a confluence-junction portion on the opposite side of the melt which separates from the gate into two groups in the circumferential direction. Hence, cold shut does not occur, and separation at the bonding portion does not occur after molding.

In order to obtain the solid-liquid coexisting state, the melt is cooled from the molten state to a semi-solidified range, thus achieving the solid-liquid coexisting state. However, the present invention is not limited to this. For example, a salt mixture in the solid phase may be heated to a semi-molten range so that a solid-liquid coexisting state is obtained. A solid powder salt (salt mixture) may be added to a molten salt so that a solid-liquid coexisting state is obtained. Alternatively, a molten salt may be added to a preheated solid salt (salt mixture) so that a solid-liquid coexisting state is obtained.

In the above description, a case in which sodium carbonate, potassium carbonate, sodium chloride, and potassium chloride are mixed such that the molar ratio XK⁺ (=[K⁺]/([Na⁺]+[K⁺])×100) of potassium ions in the entire cations is 33 mol % and that the molar ratio YCO₃ ²⁻ (=[CO₃ ²⁻]/([CO₃ ²⁻]+[Cl⁻])×100) of carbonate ions in the entire anions is 67 mol % is described. However, the present invention is not limited to this. For example, when the salt mixture has any one of the compositions shown in the following Tables 1 and 2, casting using a melt in a semi-solidified state is possible. In any case, the salt mixture is formed of only potassium ions, sodium ions, chloride ions, and carbonate ions.

Table 1 also shows the measurement results (maximum bending loads) of the bending strengths of the fabricated test pieces, and Table 2 also shows the measurement results (maximum bending strengths) of the bending strengths of the fabricated test pieces. Tables 1 and 2 are identical except that representations of the measurement results are different. The state of the bending load and that of the bending strength are shown in FIGS. 6A to 6C and FIGS. 7A and 7B in the form of bar graphs. The concentration of each ion is measured according to the analysis method determined by the rules of ion chromatograph analysis of JIS standard K0127.

TABLE 1 Cation Ratio Anion Ratio Liquidus Molding Bending Load N Sample mol % mol % Temperature Temperature 1st 2nd 3rd Number XNa⁺ XK⁺ YCl⁻ YCO₃ ²⁻ ° C. ° C. Time Time Time 1 67 33 33 67 647 638 2566 2947 2574 2 70 30 40 60 649 632 3229 3192 3274 3 70 30 30 70 665 648 3430 3158 2916 4 60 40 40 60 615 597 3021 2190 2382 5 60 40 30 70 630 619 2150 2662 2606 6 100 0 50 50 675 648 2852 4149 3322 7 100 0 30 70 753 740 3037 2535 3108 8 80 20 50 50 645 625 2526 2566 2350 9 80 20 30 70 704 678 2662 2606 2606 10 70 30 60 40 605 588 2105 3067 3177 11 50 50 30 70 604 592 2566 2347 2268 12 60 40 20 80 652 642 2290 1295 2338 13 90 10 70 30 652 635 2670 818 787 14 75 25 70 30 575 572 2424 2532 1460 15 40 60 40 60 575 565 779 668 904 16 100 0 10 90 827 821 970 1126 1112 17 60 40 10 90 685 671 1474 1533 1630 18 33 67 67 33 648 638 2048 1901 1609 19 40 60 60 40 620 606 1002 1769 1402 20 40 60 70 30 643 630 1426 1763 1410 21 30 70 70 30 655 638 1897 1345 1850 22 30 70 60 40 630 620 1354 657 1096 23 50 50 50 50 590 575 1208 910 1243 24 50 50 80 20 630 622 1394 231 821 25 40 60 80 20 652 644 424 348 625 26 60 40 70 30 602 589 1761 1553 1152 27 20 80 50 50 595 588 1290 1368 1135

TABLE 2 Cation Ratio Anion Ratio Liquidus Molding Bending Strength MPa Sample mol % mol % Temperature Temperature 1st 2nd 3rd Number XNa⁺ XK⁺ YCl⁻ YCO₃ ²⁻ ° C. ° C. Time Time Time 1 67 33 33 67 647 638 21.4 24.6 21.4 2 70 30 40 60 649 632 26.9 26.6 27.3 3 70 30 30 70 665 648 28.6 26.3 24.3 4 60 40 40 60 615 597 25.2 18.3 19.9 5 60 40 30 70 630 619 17.9 22.2 21.7 6 100 0 50 50 675 648 23.8 34.6 27.7 7 100 0 30 70 753 740 25.3 21.1 25.9 8 80 20 50 50 645 625 21.0 21.4 19.6 9 80 20 30 70 704 678 22.2 21.7 21.7 10 70 30 60 40 605 588 17.5 25.6 26.5 11 50 50 30 70 604 592 21.4 19.6 18.9 12 60 40 20 80 652 642 19.1 10.8 19.5 13 90 10 70 30 652 635 22.3 6.8 6.6 14 75 25 70 30 575 572 20.2 21.1 12.2 15 40 60 40 60 575 565 6.5 5.6 7.5 16 100 0 10 90 827 821 8.1 9.4 9.3 17 60 40 10 90 685 671 12.3 12.8 13.6 18 33 67 67 33 648 638 17.1 15.8 13.4 19 40 60 60 40 620 606 8.4 14.7 11.7 20 40 60 70 30 643 630 11.9 14.7 11.7 21 30 70 70 30 655 638 15.8 11.2 15.4 22 30 70 60 40 630 620 11.3 5.5 9.1 23 50 50 50 50 590 575 10.1 7.6 10.4 24 50 50 80 20 630 622 11.6 1.9 6.8 25 40 60 80 20 652 644 3.5 2.9 5.2 26 60 40 70 30 602 589 14.7 12.9 9.6 27 20 80 50 50 595 588 10.8 11.4 9.5

FIG. 8 shows the relationship (Phase diagram of the Na—K—Cl—CO₃ system) among the cationic ratio of potassium ions, the anionic ratio of carbonate ions, and the melting temperature (liquidus temperature). FIG. 8 shows the correspondence between the respective compositions shown in Table 1 and sample numbers. FIG. 8 also shows the liquidus temperature of NaCl when K⁺ is 0 mol % and CO₃ ²⁻ is 0 mol %, that of KCl when Na⁺ is 0 mol % and CO₃ ²⁻ is 0 mol %, that of Na₂CO₃ when K⁺ is 0 mol % and Cl⁻ is 0 mol %, and that of K₂CO₃ when Na⁺ is 0 mol % and Cl⁻ is 0 mol %. In FIG. 8, thick lines represent eutectic lines.

As is apparent from Tables 1 and 2, FIGS. 6A to 6C, FIG. 7A, FIG. 7B, and FIG. 8, the bending test results exhibit high bending strengths in a region where XK⁺ is 0 to 50 mol % and YCO₃ ²⁻ is 30 to 80 mol %. Also, the bending test results exhibit particularly high bending strengths in a region where XK⁺ is 0 to 40 mol % and YCO₃ ²⁻ is 50 to 70 mol %.

Measurement of the bending strength will now be described. To measure the bending strength, a prismatic test piece with a predetermined size is fabricated. A load is applied to the test piece, and the bending load is obtained from the maximum load needed to break the test piece. Fabrication of the test piece will be described first. A rod-like test piece 901 as shown in FIGS. 9A and 9B is formed using a predetermined metal mold. The employed metal mold is made of chrome molybdenum steel, e.g., SCM440H. FIG. 9A also shows riser portions 902 used when charging the metal mold with a melt in a semi-solidified state. In measurement of the bending strength, the portions 902 are cut off. FIG. 9A is a side view, and FIG. 9B is a sectional view taken at the position b-b in FIG. 9A. The sizes indicated in FIGS. 9A and 9B are design values of the metal mold.

To measure the bending strength of the rod-like test piece 901 fabricated in the above manner, first, as shown in FIG. 10, the test piece 901 is supported by two support portions 1001 arranged at the center of the test piece 901 at a gap of about 50 mm from each other. In this support state, at the intermediate portion of the two support portions 1001, two load portions 1002 at a gap of about 10 mm from each other apply a load to the test piece 901. The load to be applied to the test piece 901 is gradually increased. The load applied when the test piece 901 was broken was the bending load shown in Table 1.

A bending strength σ (MPa) can be obtained from a bending load P in accordance with an equation σ=3LP/BH² where H is the length of the load direction in the section of the test piece, B is a length perpendicular to the load direction in the section of the test piece, and L is the distance from the support portions 1001 serving as fulcrums to the load portions 1002 where the load acts. Although the test piece 901 is formed by pouring the melt in the solid-liquid coexisting state into the metal mold, it is difficult to form a test piece completely free from flow marks or shrinkage cavity and having a shape completely coinciding with the size true to the mold. Therefore, the bending strength is calculated based on an approximation that the test piece has a rectangular section and that H≈20 mm, B≈18 mm, and L=20 mm. Due to this approximation, the estimated strength is lower than the actual strength by approximately 0% to approximately 20%. For example, it can be assumed that a test piece which is broken by a bending load of 1200N is stronger than an ideal test piece having a bending strength of 10 MPa.

A method of manufacturing another expendable salt core according to another preferred embodiment of the present invention will be described. The following description exemplifies a case in which a mold (metal mold) is charged with a melt under a pressure to manufacture an expendable salt core 2 (die casting). A close-packed alumina crucible made of the same material as that of a Tamman tube is employed as the crucible. A predetermined amount of salt mixture consisting of sodium carbonate, potassium carbonate, sodium chloride, and potassium chloride is put in the crucible, the crucible is placed in a heating furnace, and the temperature is raised. For the purpose of protection of the crucible, the temperature is raised gradually to reach the target temperature in about 14 hours.

The target temperature is set at a value higher than the liquidus temperature corresponding to the molar ratio of the salt mixture by about 10 to about 30° C. Once the temperature reaches the target temperature, it is held at the target temperature. The temperatures of the metal mold and injection sleeve are set to approximately 180 to approximately 220° C. As the metal mold, one that can be heated to a mold temperature of about 250° C. is preferable. Also, a metal mold is preferred the casting cavity of which can be eliminated by applying a casting injection pressure of as high as about 120 MPa at maximum.

Subsequently, the melt of the salt mixture which is molten in the crucible is dipped up with a dipper. Prior to dipping up, the dipper must be heated to about 500° C. to about 600° C. by a heating device such as a burner. As soon as the melt in the crucible is dipped up with the dipper, it is started to be gradually deprived of heat by the dipper. Thus, the temperature of the melt decreases to be lower than the liquidus temperature, thus providing the solid-liquid coexisting state. During dipping up, the melt is stirred as it is shaken in the moving dipper, and primary crystals are deposited to form particles. In this manner, during the process of transporting the melt from the crucible and pouring it into the injection sleeve, the melt of the salt mixture in the dipper is set in the solid-liquid coexisting state.

When pouring the melt of the salt mixture in the semi-solidified state into the injection sleeve in this manner, the semi-solidified state progresses in the sleeve as well. Subsequently, the melt is injected into the cavity under high pressure. After charging with the melt, the casting pressure is continuously applied into the mold. For example, a pressure of about 120 MPa in the pressure ratio of a hydraulic cylinder which advances the plunger is applied into the mold. In this process, the plunger is advanced to replenish solidification shrinkage that takes place during solidification, thus continuously applying the pressure of about 120 MPa. The solidification time is about 65 to about 75 sec. In the solidification process, the plunger is continuously moved forward as long as solidification shrinkage can be replenished, thus continuously applying the pressure of about 120 MPa.

After charging the mold with the melt under the pressure and solidifying the melt in the above manner, the solidified expendable salt core is taken out from the mold. A push pin and return pin may be placed in the stationary mold so that when the mold is opened, the obtained salt core is released from the stationary mold well. The taken-out expendable salt core may be gradually cooled, and the cooled expendable salt core may be put in a dried container.

An example will be described regarding the manufacturing conditions and strength measurement result of the expendable salt core which is manufactured by injecting into a metal mold under a high pressure a melt of a salt mixture in a semi-solidified state in which the solid phase and liquid phase coexist.

The conditions are as follows:

(1) The test piece subjected to strength measurement had an almost rectangular solid shape in the same manner as in FIGS. 9A and 9B.

(2) The melt was fabricated by mixing sodium carbonate, potassium carbonate, sodium chloride, and potassium chloride and melting them. The resultant melt was adjusted such that the molar ratio XK⁺ (=[K⁺]/([Na⁺]+[K⁺])×100) of potassium ions in the entire cations was 30 mol % and that a molar ratio YCO₃ ²⁻ (=[CO₃ ²⁻]/([CO₃ ²⁻]+[Cl⁻])×100) of carbonate ions in the entire anions was 54 mol %.

(3) The liquidus temperature of the salt mixture is 630° C.

(4) The salt mixture contained in the crucible was dissolved by gradually raising the temperature to exceed the liquidus temperature of 630° C. in 14 hours. Then, the dissolved melt was held at 640 to 660° C. The temperature was controlled automatically.

(5) The dipper was heated to about 500° C. to about 600° C.

(6) The melt was dipped up with the dipper and cooled in the dipper to about 630° C. or less, so that the melt was set in the semi-solidified state.

(7) The sleeve temperature and metal mold temperature were about 180° C. to about 220° C.

(8) When the temperature of the melt of the salt mixture was 620° C. in the injection sleeve, the melt was injected into the metal mold under a high pressure as indicated by an injection curve to be described later. When the temperature of the melt of the mixture salt is 620° C., the melt is in the semi-solidified state in which the solid and liquid coexist.

To measure the pressure in the cavity, the pressures acting on the push pins provided to two portions, i.e., a gate portion 1101 shown in FIG. 11 and a portion 1102 inside the mold, were measured. The measured pressures were both about 60 MPa, as represented by the injection curves in FIG. 12. In FIG. 12, the solid line represents the measurement result obtained at the gate portion 1101, and the dashed line represents the measurement result obtained at the portion 1102. Since the injection start time point until immediately before (by about 5 sec) the solidification end point when the mold was opened, the pressures to be measured were about 60 MPa, thus maintained almost the same state. Then, at the mold opening time point, the pressures sharply decreased. Actually, the pressures to be measured gradually decrease, as shown in FIG. 12. This may be because the expendable salt core solidifies starting with its surface to interfere with the pressure from being transmitted well. A directional solidification state was exhibited in which the pressure inside the mold dropped prior to the pressure at the gate portion. As described above, the pressure of about 120 MPa was applied to the plunger. a portion of the melt solidified into gel in the injection sleeve interfered with driving of the plunger. Accordingly, the pressure actually acting on the melt in the cavity was approximately 60 MPa.

In die casting of a metal, such as aluminum, the melt has high thermal conductivity and requires a short solidification time. Hence, that portion of the melt which is located at the intermediate portion of the mold often solidifies before that portion of the melt which is located at the distal end of the mold. Then, the distal end of the mold may not be sufficiently replenished with the melt. In contrast to this, a molten salt has low thermal conductivity and requires a solidification time about three times that of aluminum. Accordingly, as shown in FIG. 12, an almost constant pressure can be continuously applied to the entire cavity until mold opening. In this manner, to apply a pressure evenly to the cavity until opening the mold, e.g., to always apply the same pressure to the cavity until mold opening or gradually change the pressure to apply to the cavity with the same change amount until mold opening, is the condition necessary to obtain a high strength.

Test pieces manufactured as described above were subjected to bending strength measurement in the same manner as described above. A high strength exceeding about 40 MPa was obtained as shown in the following Tables 3 and 4. In general, a bending strength of about 20 MPa to about 37 MPa is obtained by a widely employed expendable salt core which is manufactured by sintering after press molding (see, for example: U.S. Pat. No. 3,963,818). According to this preferred embodiment, a higher bending strength is obtained. With an expendable salt core which is manufactured by sintering after press molding, a complicated shape such as a water jacket cannot be formed. According to this preferred embodiment, however, an expendable salt core with a complicated shape can be manufactured easily. As the expendable salt core of this preferred embodiment is formed by solidifying a molten salt, the surface state of the expendable salt core reflects the surface state of the metal mold, so that a smooth surface can be obtained. Therefore, in a cast product using the expendable salt core according to this preferred embodiment, a portion which is in contact with the expendable salt core is formed to be highly smooth.

TABLE 3 Cation Ratio Anion Ratio Liquidus Molding Bending Load N Sample mol % mol % Temperature Temperature 1st 2nd 3rd Number XNa⁺ XK⁺ YCl⁻ YCO₃ ²⁻ ° C. ° C. Time Time Time 28 70 30 67 54 630 620 4812 5251 5008

TABLE 4 Cation Ratio Anion Ratio Liquidus Molding Bending Strength MPa Sample mol % mol % Temperature Temperature 1st 2nd 3rd Number XNa⁺ XK⁺ YCl⁻ YCO₃ ²⁻ ° C. ° C. Time Time Time 28 70 30 67 54 630 620 40.1 43.8 41.7

Although a salt mixture of sodium carbonate, potassium carbonate, sodium chloride, and potassium chloride is used in the above description, the present invention is not limited to this. For example, potassium carbonate, sodium chloride, and potassium chloride may be mixed, or sodium carbonate, sodium chloride, and potassium chloride may be mixed. Alternatively, other salts such as sodium bromide, potassium bromide, sodium iodide, potassium iodide, calcium chloride, potassium nitrate, sodium nitrate, potassium sulfate, lithium sulfate, magnesium sulfate, sodium sulfate, barium carbonate, and calcium carbonate may be mixed. A reinforcing ceramic material or any other reinforcing agent may be contained in the mixture.

Another example of how an expendable salt core according to a preferred embodiment of the present invention is used will be described with reference to FIGS. 13 and 14. FIG. 13 is a partially cutaway perspective view of a cylinder block which is cast using an expendable salt core for casting according to a preferred embodiment the present invention. Referring to FIG. 13, reference numeral 1301 denotes an engine cylinder block which is made of, for example, an aluminum alloy and cast using an expendable salt core 1302 as an expendable salt core according to a preferred embodiment the present invention. The expendable salt core 1302 is manufactured in the same manner as the expendable salt core 2 shown in FIG. 1. The cylinder block 1301 is preferably a portion of a water cooling 4-cycle 1-cylinder engine for a motorcycle which is molded into a predetermined shape by die casting.

The cylinder block 1301 shown in FIG. 13 comprises a cylinder bore 1303 and a cylinder body 1304 having the cylinder bore 1303. Although not shown, a crank case is attached to the lower portion of the cylinder body 1304 and rotatably, axially supports a crank shaft through a bearing.

The cylinder body 1304 is a so-called closed-deck-type cylinder body, and has a water jacket 1306 which is provided in the cylinder body 1304 using the expendable salt core 1302. The water jacket 1306 is arranged to include a cooling water passage forming portion (not shown), a cooling water inlet port (not shown), a main cooling water passage 1309, and a communication passage 1310. The cooling water passage forming portion projects on one side of the cylinder body 1304. The cooling water inlet port is provided in the cooling water passage portion. The main cooling water passage 1309 is arranged to communicate with a cooling water supply passage (not shown) provided in the cooling water passage forming portion and cover the cylinder bore 1303. The communication passage 1310 extends upward in FIG. 13 from the main cooling water passage 1309 and opens to a mating surface 1304 a with respect to a cylinder head (not shown) at the upper end of the cylinder body 1304.

The water jacket 1306 described above supplies cooling water flowing in from the cooling water inlet port (not shown) to the main cooling water passage 1309 around the cylinder bore through the cooling water supply passage, and guides the cooling water from the main cooling water passage 1309 to a cooling water passage in the cylinder head (not shown) through the communication passage 1310. Since the water jacket 1306 is provided in this manner, the cylinder body 1304 is covered with the ceiling wall (the wall that forms the mating surface 1304 a) of the cylinder body 1304 except that the communication passage 1310 of the water jacket 1306 opens to the mating surface 1304 a at the upper end to which the cylinder head is connected. Hence, a closed-deck-type arrangement is provided.

The expendable salt core 1302 to form the water jacket 1306 has a shape identical to that obtained by integrally connecting the respective portions of the water jacket 1306, as shown in the photograph of FIG. 14 as well. In FIG. 13, the cylinder body 1304 is partly cut away to facilitate understanding of the shape of the expendable salt core 1302 (the shape of the water jacket 1306). Note that reference numeral 1311 denotes a passage for a cam shaft driving chain; and reference numeral 1312 denotes a chain tensioner attaching hole.

The expendable salt core 1302 shown in FIGS. 13 and 14 is formed using a plurality of types of salts such as sodium carbonate, sodium chloride, and potassium chloride by, e.g., die casting in a solid-liquid coexisting state such as a semi-solidified state into the shape of the water jacket 1306, in the same manner as the expendable salt core 2 described above. Note that the expendable salt core 1302 can be formed by a casting method other than die casting, e.g., gravity casting. In formation of the expendable salt core 1302 which employs die casting, first, a mixture consisting of a plurality of salts (to be described later) is melted by heating to obtain a melt. Then, the temperature of the melt is decreased to set the melt in a semi-solidified (solid-liquid coexisting) state. The melt in the semi-solidified state is injected into a metal mold for an expendable salt core under a high pressure and solidified. After solidification, the obtained expendable salt core is taken out from the metal mold.

As shown in FIG. 13, in the expendable salt core 1302, the cooling water passage forming portion (not shown) which forms the cooling water inlet port and cooling water supply passage, an annular portion 1302 b which surrounds the cylinder bore 1303, and a plurality of projections 1302 a extending upward from the annular portion 1302 b are formed integrally. The projections 1302 a form the communication passage 1310 of the water jacket 1306. As is conventionally known, the expendable salt core 1302 is supported at a predetermined position in the metal mold (not shown) by a core print (not shown in FIG. 13) during casting, and is removed by dissolution with hot water or vapor after casting.

To remove the expendable salt core 1302 after casting, the cylinder block 1301 may be dipped in a dissolution tank (not shown) which contains dissolving liquid including of hydrochloric acid, hot water, or the like. When dipping the cylinder block 1301 in the dissolving liquid, the cooling water inlet port of the cooling water passage portion (not shown) and the projections 1302 a exposed to the mating surface 1304 a, of the expendable salt core 1302 come into contact with the dissolving liquid and dissolve. The dissolved portions expand gradually until all the portions dissolve finally. In this core removing process, to promote dissolution of the expendable salt core 1302 left in the water jacket 1306, hot water or vapor may be sprayed from a hole under pressure. In the expendable salt core 1302, in place of the projections 1302 a, a core print may be inserted in portions where the projections 1302 a are to be provided.

As described above, according to the present invention, the annular expendable salt core 1302 can be formed easily. Note that the region of the core print shown in the photograph of FIG. 14 is a region that projects upward from the mating surface 1304 a of FIG. 13. Although the overflow, gate, runner, and biscuit portions, shown in the photograph of FIG. 14, remain when casting the expendable salt core 1302, they are removed when using the expendable salt core 1302 for casting the cylinder block 1301.

Preferred embodiments of the present invention can be suitably used as a core for a casting such as aluminum die casting.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

1-13. (canceled) 14: A method of manufacturing an expendable salt core comprising: a first step of heating a salt mixture containing at least a potassium salt and a sodium salt to form a melt in a solid-liquid coexisting state in which a solid phase and a liquid phase coexist; a second step of charging said melt in the solid-liquid coexisting state into a core mold; and a third step of solidifying the melt in the mold to mold an expendable salt core for casting. 15: A method of manufacturing an expendable salt core according to claim 14, wherein: in the first step, the salt mixture is heated to be set in a liquid-phase-only state, and thereafter the salt mixture is cooled, thus forming the melt in the solid-liquid coexisting state. 16: A method of manufacturing an expendable salt core according to claim 14, wherein: in the second step and the third step, the mold is charged with the melt under pressure and the melt is solidified. 17: A method of manufacturing an expendable salt core according to claim 16, wherein: the pressure is applied evenly until the mold is opened. 18: A method of manufacturing an expendable salt core according to claim 14, wherein: the salt mixture is formed of only potassium ions, sodium ions, chloride ions, and carbonate ions; and a molar ratio of the potassium ions in all the cations is about 50 mol % at maximum and a molar ratio of carbonate ions in all the anions is about 30 mol % to about 80 mol %. 19: A method of manufacturing an expendable salt core according to claim 18, wherein: in said salt mixture, the molar ratio of the potassium ions in all the cations is 40 mol % at maximum and the molar ratio of carbonate ions in all the anions is 50 to 70 mol %. 20: A method of manufacturing an expendable salt core, the method comprising: molding an expendable salt core by heating a salt mixture containing at least a potassium salt and a sodium salt to form a melt in a solid-liquid coexisting state in which a solid phase and a liquid phase coexist; charging the melt in the solid-liquid coexisting state into a core mold; and solidifying the melt in the mold. 21: The method of manufacturing an expendable salt core according to claim 20, wherein: the melt in the solid-liquid coexisting state is formed by heating the salt mixture to be set in a liquid-phase-only state, and thereafter cooling the salt mixture. 22: The method of manufacturing an expendable salt core according to claim 20, wherein: the expendable salt core is formed by charging the mold with the melt under a pressure and then solidifying the melt. 23: The method of manufacturing an expendable salt core according to claim 22, wherein: the pressure is applied evenly until the mold is opened. 24: The method of manufacturing an expendable salt core according to claim 20, wherein: the salt mixture is formed of only potassium ions, sodium ions, chloride ions, and carbonate ions; and a molar ratio of the potassium ions in all cations is about 50 mol % at maximum and a molar ratio of carbonate ions in all the anions is about 30 mol % to about 80 mol %. 25: The method of manufacturing an expendable salt core according to claim 24, wherein: the molar ratio of the potassium ions in all the anions is about 40 mol % at maximum and the molar ratio of carbonate ions in all the cations is about 50 mol % to about 70 mol % in the salt mixture. 26: The method of manufacturing an expendable salt core according to claim 20, wherein: the expendable salt core includes a core to mold a water jacket arranged to water cool a closed-deck type cylinder block of engine. 